In general, solid state light sources (e.g., semiconductor laser or LED modules) are known to have an impedance which is different from that of the driver circuits (hereinafter drivers) to which they are electronically coupled. This impedance mismatch means that some of the power delivered by the driver is wasted. In many systems applications, such as analog systems for wireless communications, the power budget does not tolerate wasting any significant amount of power. For example, semiconductor laser modules for use in wireless PCS or cellular systems typically have an impedance of about 7 .OMEGA., whereas their drivers typically have a much higher impedance in the range of about 25-75 .OMEGA.. Furthermore, specifications are often placed on the RF input impedance of a laser module, either in terms of the impedance itself or in terms of the return loss associated with the characteristic impedance of measurement systems used to test the module. Required RF impedance levels are also typically in the range of about 25-75 .OMEGA..
In the prior art, this problem is sometimes addressed by adding a resistor in series with the laser. Although this approach is simple, low cost and broadband, most of the driver power is dissipated in the series matching resistor, rather than being used for its intended function--to generate higher optical output power from the laser. Wasting power in this fashion is of concern in any system where efficiency is important, especially in wide bandwidth applications. Another prior art design uses an electrical transformer between the laser and the driver. Although this approach is also simple and low cost, it may suffer disadvantages associated with the limited bandwidth and lack of parasitic control in the transformer.
Thus, there is a need remaining in the art for an impedance matching scheme which addresses these shortcomings.